This invention relates to certain improvements in apparatus and methods for melting glass. More particularly, this invention relates to apparatus and methods which control the location of the "hot spot," i.e. area of highest temperature in the liquid pool of melting or molten glass in a glass melter so as to control the wear out of various melter and discharge elements thereby reducing the number of shutdowns needed for replacement or rebuild purposes. Still further, this invention relates to unique methods and apparatus for venting corrosive volatiles from the system.
Melters of various shapes and sizes which present glass batch (usually in powdered ingredient form, with or without cullet) often by floating the batch material as a relatively thick layer on top of a molten pool of glass being heated and melted beneath the batch, and thereafter distributing the molten glass from the pool through a discharge port in a side wall of the melter to a conditioning zone (conditioner), and thereafter to a forehearth array or other working area, are well known in the art. Exemplary of such systems are conventional, in line combinations of a melter, conditioner, and forehearth used to distribute molten glass to an array of spinners for making fiberglass batts of insulation. Other uses for such combinations are, of course, known, and the art, as a whole, is generally represented by the following prior art references:
U.S. Pat. Nos. 3,498,779 4,365,987 3,897,234 4,812,372 4,001,001 4,994,099 4,017,294 5,194,081 4,023,950 5,616,994.
Generally speaking, and prior to my invention in the aforesaid application Ser. No. 08/917,207, now U.S. Pat. No. 5,961,686, the art of glass making accepted the problem of multiple shutdowns due to the fact that the various elements in conventional melters, except in very unusual and unpredictable situations, wore out at different times. In this respect, it is characteristic in the prior art construction of melters to employ a cylindrical or rectangular tank-like configuration in which the side and bottom walls are formed of refractory material such as Cr, Al--Zr--Si, or Al/Cr based compositions whose corrosion rate usually increases with increased temperatures. Adding to this problem is the fact that in such configurations one or more discharge ports are either required or desired at different locations within the tank, e.g. in the bottom wall and in at least one location in the side wall of the tank. Because the temperature of the glass can, and often does, differ markedly between a "hot spot" volume in the molten glass, usually in the center of the tank near the bottom wall, and the remaining molten glass volume, e.g. at the side walls, melter parts in the cooler areas wear out less rapidly than parts located in or proximal the "hot spot."
In a typical example of this problem, the melting tank is provided in its bottom wall with a discharge port for draining the tank and a side discharge port for distributing the molten glass to a conditioning zone. Such discharge ports, whether in the bottom or side walls, are normally formed of molybdenum or an alloy thereof which is relatively corrosion resistant and thus is reasonably able to withstand the high temperatures experienced in the melter over a given period of time. Unfortunately, like the refractory wall material, these molybdenum based ports have a corrosion rate which increases with temperature.
In many melters it has also been conventional to cool the walls by various techniques such as with a water-cooled shell surrounding the melter. Such cooling of the bottom and side walls, despite inherent currents of flow in the molten glass, tend to isolate the "hot spot" and set up the temperature differentials as discussed above, which then lead to the differences in wear out rates of the various parts and the need for expensive, time consuming, multiple shutdowns otherwise unnecessary if all the parts were to wear out at substantially the same time.
In a typical prior art melter, for example, usually of a circular, cylindrical bottom, side wall configuration, the furnace is open topped, side and bottom wall cooled, and is provided with electrodes to melt the batch material. These electrodes are usually located in the melter either above the batch or in the molten pool of glass itself, often near the bottom or inserted through the batch. Powdered batch material is then "floated" on top of the melting glass beneath it, usually by a conventional, metered batch delivery system located above the melt area and fed by gravity continuously to the batch layer as its underneath surface melts into the molten volume of glass beneath it. It is, of course, within this molten glass volume beneath the batch layer that the aforesaid "hot spot" forms.
While convection currents created in the melting glass serve to equalize, somewhat, the temperature of the molten glass pool, it is very often an inherent characteristic of such melters, particularly where bottom entry electrodes are employed, that the bottom center of the melter is where the "hot spot" forms. For example, a typical "hot spot" may be from about 3150.degree.-3250.degree. F. By contrast, the side walls will only then be, particularly if water-cooled, at a significantly lower temperature, e.g. about 2500.degree.-2700.degree. F. Even if water-cooled, in certain instances, the bottom wall will be so close to the "hot spot" that its temperature in a localized area will, for all intents and purposes, be that of the "hot spot," thus differing from other areas of the bottom wall, as well as the side wall and discharge port in the side wall. Since the drainage port is conventionally located in the center of the bottom wall, and thus at or proximal the usual "hot spot" location, its corrosion rate differs markedly from that of the side discharge port and side walls.
As exemplified by the above typical melter arrangement, multiple melter shutdowns may thus become necessary. For example, the discharge drain port and/or bottom wall may have to be replaced, while the side walls and side discharge port remain in acceptable operating condition, only to have to replace one or more of these two latter parts at a later time in a second shutdown, while the replaced bottom wall and/or drain discharge port are not yet worn sufficiently to economically justify their replacement.
In short, it would constitute a considerable advance in the art of glass melting if a technique were developed which could control the location of the aforesaid "hot spot" in a glass melter so as to displace it (locate it) away from the refractory walls and metallic discharge port tubes (side and bottom) such that all of the elements in the melter subject to corrosion and wear out therefrom were to wear out at substantially the same time.
The term "at substantially the same time," as used herein, means that the elements which are the subject of corrosive wear out are in such a condition at the time that one element is in the most advanced condition of wear out, that it is economically justifiable to replace all the elements, rather than to go through another shutdown to replace a less worn out element when it completely wears out later in time.
In my aforesaid co-pending application Ser. No. 08/917,207, now U.S. Pat. No. 5,961,686, the disclosure of which is incorporated herein by reference, a significant advance toward reaching this goal and solving this prior art problem was achieved, based upon the acceptance of the inherent location of the "hot spot" in the melter. By the use of a unique discharge port concept located in the side wall of the melter, a sufficient distance away from the "hot spot," coupled optionally with side and bottom wall cooling means, the side discharge ports and side walls could justifiably be replaced at the same time. In addition, in certain embodiments, by relying on convection currents and sufficient bottom wall cooling, the bottom wall and bottom discharge drain theoretically could, at times, be controlled to wear out at substantially the same time as the side wall and side discharge port. Despite this significant advance in the art, it has now been found that the "hot spot" (i.e. volume of highest temperature) often exists, in certain furnaces, inherently too close to the bottom and/or side walls of the melter tank and that circulation currents, even with wall cooling, are insufficient to keep the wear out rate of the bottom wall and bottom discharge orifice truly substantially equal to that of the side walls and side discharge orifice. Thus there continues to be a need in the art for a still further improvement which creates an even more equalized wear out rate among the essential parts in the melter (e.g. the refractory melter lining which makes up the melter walls, side and bottom, and the various discharge ports in these walls.
There is yet another problem which the art of glass melting has had to face. In many glass melting operations, such as in melting glass ultimately used to make fiberglass insulation, it is necessary to employ batch ingredients which create highly corrosive volatiles during the melting and/or conditioning operation. These volatiles often end up in the atmosphere above the glass and can thus rapidly corrode walls, orifices and heater elements if not effectively exhausted from the system. Such volatiles are well known and include, for example, various sodium and borate compounds.
In melting systems which do not employ, or need not employ, the highly advantageous technique during glass melting of floating batch material in a relatively thick layer (e.g. about 3"-4", or at times as high as 10") on top of the molten glass, the corrosive volatiles can usually be exhausted during melting by exhausting them from the melter itself. However, when the more desirable batch technique of floating the batch on top of a molten pool of glass is employed, the volatiles do not readily escape through the batch, but rather are only released in the conditioner when the molten glass is then freed from the batch material being on top of it. This, then, gives rise to the need for a new technique for effectively eliminating corrosive volatiles from the glass in the conditioner, particularly before they reach the forehearth.
It is a purpose of this invention to fulfill the above needs in the art, as well as other needs which will become apparent to the skilled artisan once given the following disclosure.